Aromatic polyesters typified by polyethylene terephthalate and polyethylene-2,6-naphthalate (polyethylene-2,6-naphthalene dicarboxylate) are widely used in films because they have excellent mechanical properties, dimensional stability and heat resistance. Especially polyethylene-2,6-naphthalate has more excellent mechanical properties, dimensional stability and heat resistance than polyethylene terephthalate and is therefore used in fields in which these requirements are very high, for example, base films for high-density magnetic recording media. However, the requirement for dimensional stability in the high-density magnetic recording media is becoming higher and higher, and the further improvement of this property is desired.
Patent Documents 1 to 5 propose a polyalkylene-6,6′-(alkylenedioxy)di-2-naphthoate obtained from diethyl-6,6′-(alkylenedioxy)di-2-naphthoate which is an ester compound of 6,6′-(alkylenedioxy)di-2-naphthoic acid.
Especially Patent Document 3 teaches that a magnetic recording flexible disk having a small tracking shift is obtained from a film of polyethylene-6,6′-(ethylenedioxy)di-2-naphthoate by setting its maximum temperature expansion coefficient to 10 to 35 (ppm/° C.), its maximum humidity expansion coefficient to 0 to 8 (ppm/% RH), the difference between the maximum and minimum temperature expansion coefficients to 0 to 6.0 (ppm/° C.) and the difference between the maximum and minimum humidity expansion coefficients to 0 to 4.0 (ppm/% RH).
However, the requirement for the improvement of recording density in magnetic recording media is now very high and accordingly, dimensional stability required for the base film cannot be attained not only by polyethylene terephthalate but also by polyethylene-2,6-naphthalate and the film proposed by Patent Document 3 any longer.    (Patent Document 1) JP-A 60-135428    (Patent Document 2) JP-A 60-221420    (Patent Document 3) JP-A 61-145724    (Patent Document 4) JP-A 6-145323    (Patent Document 5) WO2008/010607    (Patent Document 6) WO2008/096612